Method of joining heat-treatable aluminum alloy members by friction stir welding

ABSTRACT

A method of joining heat-treatable aluminum alloy members by friction stir welding, including the steps of: a T4-treatment-performing step of performing a T4 treatment on heat-treatable aluminum alloy members so as to impart T4 temper to the heat-treatable aluminum alloy members; a joining step of joining the heat-treatable aluminum alloy members with T4 temper by friction stir welding to provide a joined product; and a reversion-treatment-performing step of performing a reversion treatment, the reversion-treatment-performing step being carried out prior to or after the joining step.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 11/305,547,having a filing date of Dec. 15, 2005, the entirety of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a method of joiningheat-treatable aluminum alloy members by friction stir wielding and ajoined product for press forming obtained by the method. Moreparticularly, the invention relates to such a method capable ofadvantageously providing a joined product with good ductility,accordingly with good press formability while effectively preventingoccurrence of fracture at a stir zone and heat-affected zones of thejoined product obtained as a result of joining the heat-treatablealuminum alloy members by friction stir welding, and such a joinedproduct for press forming advantageously produced by the method.

2. Discussion of Related Art

Conventionally, a plate or sheet member made of a heat-treatablealuminum alloy is used as a blank for press forming. Such a blank issubjected to the press forming, thereby producing various kinds of pressproducts. In a press working operation of the heat-treatable aluminumalloy plate member, rather than a T6-treated aluminum alloy plate member(i.e., aluminum alloy plate member with T6 temper) which assures highstrength, a T4-treated aluminum alloy plate member (i.e., aluminum alloyplate member with T4 temper) is preferably used because of its excellentductility and softness lower than the T6-treated aluminum alloy platemember.

In the meantime, in recent years, a plurality of aluminum alloy membersare joined together to provide a joined product, and the obtained joinedproduct is subjected to a press working operation, a bending operationand the like for improving a material yield as a result of eliminationof cutting and trimming after the press working operation andsimplifying the production process. More specifically described, aplurality of aluminum alloy members are butted and welded together intothe integrally joined product, thereby providing one blank for pressforming, i.e., a so-called tailored blank. A technique of performingsuitable press forming on the thus obtained blank is widely employed inthe field of a press product for an interior panel of automotivevehicles, for instance.

In addition to the improvement in the material yield and thesimplification of the production process, such a press forming techniqueprovides an advantage of utilizing, as a material for the blank, asmall-sized metal plate which cannot be used in an ordinary pressforming technique. Further, where a blank obtained by joining togetherthe heat-treatable aluminum alloy members having mutually differentthickness is used as a blank for press forming, it is easy to produce apress product having required strength at required portions thereof.

As a method of joining the heat-treatable aluminum alloy members forobtaining such a blank for press forming, various welding methods areconventionally employed such as TIG welding (tungsten inert-gaswelding), MIG welding (metal inert-gas welding), laser welding, andfriction stir welding. Among those, an increasing attention has beendrawn to the friction stir welding process which is a sort ofsolid-phase welding or joining process involving a relatively smallamount of heat generation, relatively small degrees of softening anddistortion, as compared with a fusion welding process, and thereforeassuring a sufficiently high degree of welding strength or jointstrength. However, even where the T4-treated members are butted andjoined together according to such a friction stir welding process toprovide the blank, softening inevitably occurs at heat-affected zones ofthe blank which are located adjacent to a stir zone at which theT4-treated members are welded and which are affected by a heat generatedin the friction stir welding process. If such a blank is used for pressforming in the as-welded condition without undergoing anypost-treatment, stress concentrates locally on the heat-affected zonesat which the strength (hardness) is the lowest due to the heat generatedupon welding, so that the heat-affected zones tend to deform faster thanthe other portion of the blank and often break or fracture. The fractureor breakage at the heat-affected zones raises a problem of limitation ina configuration of the press product to be obtained from the blank and adegree of elongation or ductility (a deformation amount) of the blank.

To deal with the problem indicated above, there have been made: (1)proposals to improve a joint strength of a welded region by performingan age-hardening treatment after the heat-treatable aluminum alloymembers have been welded together (as disclosed in the following PatentDocuments 1-5); and (2) proposals to improve an overall joint strengthof a joined portion including the heat-affected zones by performing anage-hardening treatment after the heat-treatable alloy members have beenare joined by friction stir welding (as disclosed in the followingPatent Documents 6-8).

The proposals (1) will be explained in detail. Patent Document 1(JP-A-5-117826) discloses a method of producing a rim made of ahigh-strength aluminum alloy comprising: forming an aluminum alloymember into a rim shape after performing flush butt welding; and heatingthe rim-shaped member at a temperature of 170-200° C. Patent Document 2(JP-A-8-246116) discloses a method of producing an aluminum-alloy madewheel with increased strength comprising: welding a disc made of anAl—Mg—Si series alloy to a rim made of an aluminum alloy; cooling thewelded region at a rate of not less than 1° C./second and subsequentlyheating at a temperature of 100-200° C. for 5-60 minutes. PatentDocument 3 (JP-A-9-177974) discloses a technique of restoring aheat-softening phenomenon by a welding heat by performing an agingtreatment which involves heating at a temperature of 150-200° C. for notless than 30 minutes after welding. Patent Document 4 (JP-A-11-199994)discloses that a welded member obtained by welding Al—Mg—Si series alloyextruded members is subjected, after quenching, to an aging treatment ata temperature of 150-220° C. for 3-24 hours, for increasing thestrength. Patent Document 5 (JP-A-2002-294381) discloses a welded jointof an excess Si-content type JIS-A 6000 series aluminum alloy obtainedby welding aluminum alloy members and having joint strength and jointelongation restored by performing an aging treatment at a temperature ofnot higher than 180° C. for about 10-50 minutes.

The proposals (2) will be explained in detail. Patent Document 6(JP-A-11-104860) discloses a welded joint with intended strengthobtained by: joining together members by friction stir welding;subjecting, immediately after the welding, a welded portion to forcedair cooling for placing the welded portion into a hardened state; andperforming a suitable artificial age-hardening treatment or naturalage-hardening treatment. Patent Document 7 (JP-A-2000-61663) discloses amethod of joining aluminum alloy members with T1 temper comprising:joining the members by friction stir welding such that a time periodduring which heat-affected zones located outwardly of a stirred regionare heated at a temperature of not lower than 300° C. is controlled tobe within one minute; and performing an aging treatment at a temperaturelower than 300° C. for about from 10 minutes to 24 hours, therebyattaining the joint strength that is not less than 95% with respect toyield strength of a base material. Patent Document 8 (JP-A-2002-346770)discloses that aluminum-based alloys whose average crystal grain size is100-5×10³ nm are joined together by friction stir welding and thensubjected to an aging treatment at a temperature of 100-200° C.

To assure a high degree of joint strength, Patent Document 9(JP-A-2001-321948) discloses a method of welding heat-treatable aluminumalloy members comprising a step of performing, prior to a weldingprocess such as TIG welding, MIG welding, laser welding and frictionstir welding, a sub-aging treatment in which an aging treatment isperformed on the aluminum alloy members at a temperature lower than andfor a time period shorter than those in aging treatment conditions inwhich maximum strength is obtained. Patent Document 9 teaches, in itsexample, a heat treatment carried out at a furnace temperature of160-200° C. for three hours as the specific conditions of such asub-aging treatment.

By performing the age-hardening treatment after fusion welding orfriction stir welding or by performing the sub-aging treatment beforewelding as disclosed in the above-indicated Patent Documents, the jointstrength is improved. Those Patent Documents, however, are silent aboutpress formability and ductility of the joined product obtained byjoining the aluminum alloy members. Accordingly, the strength of theheat-affected zones is lower than those of the welded region and thebase material, so that the stress concentrates on the heat-affectedzones, causing fracture at the heat-affected zones. Therefore, therestill exists the problem of limitation in the configuration of the pressproduct to be obtained from the joined product and the elongation degree(the formable amount) of the joined product.

Further, Patent Document 10 (JP-A-10-225781) discloses a friction stirwelding process in which, prior to friction stir welding, a portionahead of a probe as seen in its moving direction is heated at atemperature of not higher than 500° C., preferably in a range of100-300° C. using an external heat source such as a laser radiation, agas flame or a heater. According to the disclosed method, the membersare joined without suffering from any failure while improving a rate ofjoining the members and increasing a service life of the probe. PatentDocuments 11 and 12 (JP-A-2003-80381 and JP-A-2003-94175) disclose atechnique of heating portions ahead of and behind a joint region at atemperature in a rage of 100-300° C. by means of a welding torch orinduction heating for shortening a time before initiation of frictionstir welding or for preventing occurrence of cracking at the joinedregion including the heat-affected zones. In the methods disclosed inthose Patent Documents 10-12, although the heat treatment is performedon the aluminum alloy members prior to the friction stir welding, theheat treatment is performed only for reducing resistance to deformationand shortening the time before initiation of friction stir welding. Noneof these methods aim at refining the aluminum alloy members.

SUMMARY OF THE INVENTION

In the light of the aforementioned situations, the inventors of thepresent invention have made an extensive study to solve the problemsrelated to the press formability of the joined product obtained byjoining together the heat-treatable aluminum alloy members. As a resultof the study, it has been found that the concentration of stress on thestir zone and the heat-affected zones during the press forming operationcan be avoided by optimizing relationship in strength among the stirzone, the heat-affected zones and the base material, more specifically,by making hardness of the stir zone and the heat-affected zones largerthan that of the base material.

For optimizing the relationship in strength among the stir zone, theheat-affected zones and the base material, the inventors have made astudy from the metallurgical viewpoint on respective metal structures ofthe stir zone, heat-affected zones and base material portions afterfriction stir welding. The study has revealed that the heat-affectedzones located in the vicinity of the stir zone and having the lowesthardness are in a state in which the metal structure is rehardened orreverted (in a reversion state). It has been further revealed that thestrength or hardness of the base material is effectively reduced as lowas or lower than that of the heat-affected zones (a) where a reversiontreatment for temporarily extinguishing GP zones (clusters) existing inthe metal structure is performed on T4-treated heat-treatable aluminumalloy members in advance and subsequently the aluminum alloy members arejoined together by friction stir welding after the reversion treatmentor (b) where T4-treated heat-treatable aluminum alloy members are joinedtogether by friction stir welding to provide a joined product and areversion treatment for temporarily extinguishing GP zones (clusters) issubsequently performed on the joined product for reverting the metalstructure of the base material. Consequently, the fracture due to thestress concentration on the heat-affected zones in the press forming canbe advantageously avoided and the stress can be effectively distributedover the base material portion with a large area, so that the amount ofdeformation of the joined product as a whole is advantageouslyincreased. Therefore, the ductility and the breaking elongation of thejoined product can be effectively enhanced, leading to an improvement inthe press formability.

The present invention has been accomplished based on the findingsdescribed above. It is therefore an object of the invention to provide amethod of joining heat-treatable aluminum alloy members by friction stirwelding, which method is capable of providing a joined product havingexcellent ductility and accordingly excellent press formability withoutsuffering from fracture at a stir zone and heat-affected zones byoptimizing relationship in strength among the stir zone, theheat-affected zones and a base material. Another object of the inventionis to provide a joined product for press forming with enhanced pressformability advantageously produced by such a method.

To achieve the object indicated above, the present invention provides amethod of joining heat-treatable aluminum alloy members by friction stirwelding, comprising the steps of: a T4-treatment-performing step ofperforming a T4 treatment on heat-treatable aluminum alloy members so asto impart T4 temper to the heat-treatable aluminum alloy members; ajoining step of joining the heat-treatable aluminum alloy members withT4 temper by friction stir welding to provide a joined product; and areversion-treatment-performing step of performing a reversion treatment,the reversion-treatment-performing step being carried out prior to orafter the joining step.

According to the present method described above, the heat-treatablealuminum alloy members can be joined advantageously so as to provide ajoined product with excellent ductility and accordingly excellent pressformability.

The present method is classified into two methods according torespective two aspects described below.

In the method according to the first aspect, thereversion-treatment-performing step is carried out prior to the joiningstep for placing the heat-treatable aluminum alloy members in areversion state and the joining step is carried out such that theheat-treatable aluminum alloy members in the reversion state are joinedtogether by friction stir welding.

In the method according to the first aspect indicated above, theheat-treatable aluminum alloy members are subjected to the T4 treatmentand then to the reversion treatment. Accordingly, the GP zones (orclusters) which are formed in a metal structure of the aluminum alloymembers with T4 temper and which are fine phases of several atoms aretemporarily extinguished. (Hereinafter, the aluminum alloy members withT4 temper may be referred to as “T4-treated” or “T4 temper” aluminumalloy members.) Where the heat-treatable aluminum alloy members in whichthe GP zones are extinguished are butted to each other and joined byfriction stir welding, a stir zone at which the heat-treatable aluminumalloy members are joined or welded together to provide a joined producthas a structure similar to that when subjected to a solution heattreatment. The strength or hardness of the stir zone is higher than thestrength or hardness of the base material and the strength or hardnessof the heat-affected zones. The heat-affected zones have a reversionstructure which has undergone slight age-hardening. The strength orhardness of the heat-affected zones is equal to or higher than thestrength or hardness of the base material and lower than the strength orhardness of the stir zone. On the other hand, the base material whichgives non-welded portions (portions of the joined product excluding awelded region that includes the heat-affected zones) maintains thereversion state or structure and the strength or hardness of the basematerial is the lowest. As a consequence, even if the joined product issubjected to press forming at a portion thereof including a jointregion, it is possible to avoid the conventionally experiencedoccurrence of fracture at the heat-affected zones due to the stressconcentrating locally on the heat-affected zones, permitting the stressto be distributed or dispersed in the base material. Accordingly, theamount of deformation of the joined product is increased, resulting ineffectively enhanced breaking elongation or ductility. Thus, the pressformability can be advantageously improved.

In a first preferred form of the method according to the above-indicatedfirst aspect, the reversion treatment is a heat treatment in which theheat-treatable aluminum alloys with T4 temper are heated at an elevatedtemperature of 150-350° C. for a time period of not longer than 300seconds.

In a second preferred form of the method according to theabove-indicated first aspect, each of the heat-treatable aluminum alloymembers is formed of a 6000 series aluminum alloy and the reversiontreatment is a heat treatment in which the heat-treatable aluminum alloymembers with T4 temper are heated at an elevated temperature of 200-350°C. for a time period of not longer than 300 seconds.

In a third preferred form of the method according to the above-indicatedfirst aspect, each of the heat-treatable aluminum alloy members isformed of a 2000 series aluminum alloy and the reversion treatment is aheat treatment in which the heat-treatable aluminum alloy members withT4 temper are heated at an elevated temperature of 150-300° C. for atime period of not longer than 300 seconds.

In a fourth preferred form of the method according to theabove-indicated first aspect, each of the heat-treatable aluminum alloymembers is formed of a 7000 series aluminum alloy and the reversiontreatment is a heat treatment in which the aluminum alloy members withT4 temper are heated at an elevated temperature of 150-250° C. for atime period of not longer than 300 seconds.

According to the second through fourth preferred forms indicated above,the heating conditions of the reversion treatment are preciselydetermined depending upon the kind of the heat-treatable aluminum alloymembers, whereby the relationship in strength among the stir zone, theheat-affected zones and the base material can be further effectivelyoptimized. Therefore, it is possible to more advantageously obtain thejoined product with good ductility and good press formability.

In a fifth preferred form of the method according to the above-indicatedfirst aspect, the joining step is carried out on the heat-treatablealuminum alloy members in the reversion state before an increase intensile strength of the heat-treatable aluminum alloy members due tonatural aging exceeds 10 MPa.

According to the fifth preferred form indicated above, the strength orhardness of the base material can be made lower, with high reliability,than the strength or hardness of the stir zone and the strength orhardness of the heat-affected zones.

In a sixth preferred form of the method according to the above-indicatedfirst aspect, the reversion treatment is performed using a heating meansselected from a salt bath, an oil bath, an air-heating furnace, an iron,infrared heating and induction heating.

According to the sixth preferred form indicated above, the heattreatment with the intended conditions can be advantageously practiced.

In a seventh preferred form of the method according to theabove-indicated first aspect, the reversion-treatment-performing step iscarried out such that the reversion treatment is sequentially performedusing a suitable heating device along portions of the heat-treatablealloy members at which they are to be joined together by friction stirwelding and the joining step is carried out such that the portions whichhave been subjected to the reversion treatment are sequentially joinedtogether by friction stir welding.

According to the seventh preferred form indicated above, on the pair ofheat-treatable aluminum alloy members to be joined, the operation ofperforming the reversion treatment and the operation of friction stirwelding are not carried out separately at separate locations, but arecarried out sequentially and continuously on the same line. Therefore,the joined product with excellent press formability can be continuouslyand efficiently produced. Moreover, because the aluminum alloy memberswhich have been subjected to the reversion treatment are free fromage-hardening due to natural aging which occurs before the operation offriction stir welding is initiated. Therefore, the strength or hardnessof the base material can be made lower with higher reliability than thestrength or hardness of the stir zone and the strength or hardness ofthe base material.

In the method according to the second aspect, thereversion-treatment-performing step is carried out after the joiningstep so as to perform the reversion treatment on the joined productobtained in the joining step, before GP zones are formed at a stir zoneof the joined product.

According to the method according to the second aspect indicated above,the heat-treatable aluminum alloy members which have been subjected tothe T4 treatment are joined together by friction stir welding to providea joined product, and then the reversion treatment is performed on thejoined product. Accordingly, the base material of the joined productwith T4 temper is reverted, namely, the GP zones (or clusters) which areformed in the metal structure of the base material (in T4 temper) andwhich are fine phases of several atoms are temporarily extinguished,whereby the strength or hardness of the base material can be effectivelylowered. On the other hand, the stir zone having, owing to the frictionstir welding, a metal structure similar to that when subjected to asolution heat treatment maintains the metal structure even after thereversion treatment. The strength or hardness of the stir zone is higherthan the strength or hardness of the base material and the strength orhardness of the heat-affected zones. The heat-affected zones placed in areversion state by the friction stir welding has a reversion structurewhich has undergone slight age-hardening by the subsequent reversiontreatment. The strength or hardness of the heat-affected zones is equalto or higher than the strength or hardness of the base material andlower than the strength or hardness of the stir zone. As a result, evenif the joined product is subjected to press forming at a portion thereofincluding a joint region, it is possible to avoid the conventionallyexperienced occurrence of fracture at the heat-affected zones due to thestress concentrating locally on the heat-affected zones, permitting thestress to be distributed or dispersed in the base material. Accordingly,the amount of deformation of the joined product is increased, resultingin effectively enhanced breaking elongation and ductility. Thus, thepress formability can be advantageously improved.

In a first preferred form of the method according to the above-indicatedsecond aspect, the reversion treatment is a heat treatment in which thejoined product is heated at an elevated temperature of 150-350° C. for atime period of not longer than 300 seconds.

In a second preferred form of the method according to theabove-indicated second aspect, each of the heat-treatable aluminum alloymembers is formed of a 6000 series aluminum alloy and the reversiontreatment is a heat treatment in which the heat-treatable aluminum alloymembers are heated at an elevated temperature of 200-350° C. for a timeperiod of not longer than 300 seconds.

In a third preferred form of the method according to the above-indicatedsecond aspect, each of the heat-treatable aluminum alloy members isformed of a 2000 series aluminum alloy and the reversion treatment is aheat treatment in which the heat-treatable aluminum alloy members areheated at an elevated temperature of 150-300° C. for a time period ofnot longer than 300 seconds.

In a fourth preferred form of the method according to theabove-indicated second aspect, each of the heat-treatable aluminum alloymembers is formed of a 7000 series aluminum alloy and the reversiontreatment is a heat treatment in which the heat-treatable aluminum alloymembers are heated at an elevated temperature of 150-250° C. for a timeperiod of not longer than 300 seconds.

According to the second through fourth preferred forms indicated above,the heating conditions of the reversion treatment are preciselydetermined depending upon the kind of the heat-treatable aluminum alloymembers, whereby the relationship in strength among the stir zone, theheat-affected zones and the base material can be further effectivelyoptimized. Therefore, it is possible to more advantageously obtain thejoined product with good ductility and good press formability.

In a fifth preferred form of the method according to the above-indicatedsecond aspect, the reversion-treatment-performing step is carried out onthe joined product obtained in the joining step before a time periodduring which the joined product undergoes natural aging that occursafter the heat-treatable aluminum alloy members have been joined byfriction stir welding exceeds 24 hours.

According to the fifth preferred form indicated above, the strength orhardness of the base material can be made lower, with high reliability,than the strength or hardness of the stir zone and the strength orhardness of the heat-affected zones.

In a sixth preferred form of the method according to the above-indicatedsecond aspect, the reversion treatment is performed using a heatingmeans selected from a salt bath, an oil bath, an air-heating furnace, aniron, infrared heating and induction heating.

According to the sixth preferred form indicated above, the heattreatment with the intended conditions can be advantageously practiced.

In an seventh preferred form of the method according to theabove-indicated second aspect, the joining step is carried out such thatthe heat-treatable alloy members are joined together sequentially alongportions thereof at which they are to be joined by friction stir weldingand the reversion-treatment-performing step is carried out such that thereversion treatment is sequentially performed, using a suitable heatingdevice, on the joined product which at least includes a stir zone formedby friction stir welding in the joining step.

According to the seventh preferred form indicated above, on the pair ofheat-treatable aluminum alloy members to be joined, the operation offriction stir welding and the operation of performing the reversiontreatment are not carried out separately at separate locations, but arecarried out sequentially and continuously on the same line. Therefore,the joined product with excellent press formability can be continuouslyand efficiently produced. Moreover, because the joined product obtainedafter the operation of friction stir welding does not suffer fromage-hardening due to natural aging which occurs before the reversiontreatment is initiated. Therefore, the strength or hardness of the basematerial can be made lower, with higher reliability, than the strengthor hardness of the stir zone and the strength or hardness of the basematerial.

To attain the above-indicated object, the present invention provides ajoined product for press forming obtained by a method of joiningheat-treatable aluminum alloy members by friction stir welding,comprising the steps of: (a) a T4-treatment-performing step ofperforming a T4 treatment on heat-treatable aluminum alloy members so asto impart T4 temper to the heat-treatable aluminum alloy members; (b) ajoining step of joining the heat-treatable aluminum alloy members withthe T4 temper by friction stir welding to provide a joined product; and(c) a reversion-treatment-performing step of performing a reversiontreatment, the reversion-treatment-performing step being carried outprior to or after the joining step.

The joined product for press forming constructed as described above isproduced according to any of the methods described above. Accordingly,it is possible to avoid occurrence of fracture at the heat-affectedzones in press forming, resulting in an increased amount of deformationof the joined product. Consequently, the joined product assures a highdegree of freedom in the configuration of a press product to be obtainedand a larger deformation amount in press forming.

To attain the above-indicated object, the present invention provides ajoined product for press forming with excellent ductility obtained byjoining heat-treatable aluminum alloy members which have been subjectedto a T3 treatment or a T4 treatment, wherein a joint region of thejoined product at which the heat-treatable aluminum alloy members arejoined together being formed such that hardness of the joint region anda heat-affected zone located adjacent to the joint region is held in arange of 100-200 where hardness of base material portions of the joinedproduct is 100.

The joined product indicated above is advantageously produced, forinstance, according to any of the methods described above. In the joinedproduct, the relationship in strength among the joint region, theheat-affected zones and the base material is optimized, resulting inimproved ductility and formability. In addition, it is possible toincrease the hardness of the joined product by performing a heattreatment such as bake hardening after it has been subjected to aforming operation.

In the joined product indicated above, a total width of the joint regionand the heat-affected zones on a surface of the joined product ispreferably held in a range of 1 mm to 100 mm.

Preferably, in the joined product indicated above, fracture occurs atthe base material portions in a case where a tensile test is conductedusing a rectangular specimen defined in ASTM B 557-94 that is obtainedfrom the joined product such that the specimen includes, as a parallelportion thereof, the joint region and the heat-affected zones.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, advantages and technical andindustrial significance of the present invention will be betterunderstood by reading the following detailed description of presentlypreferred embodiments of the invention, when considered in connectionwith the accompanying drawings, in which:

FIG. 1 is a cross-sectional view showing one example of a process ofjoining heat-treatable aluminum (Al) alloy members by friction stirwelding, the view showing a state in which the Al alloy members arebutted to each other before welding;

FIG. 2 are views for explaining relationship in hardness among a stirzone, heat-affected zones and a base material of a joined productobtained by friction stir welding, the views including: a crosssectional view showing planar heat-treatable Al alloy members which havebeen joined by friction stir welding; a graph showing hardnessdistribution of a joined product on which the reversion treatment hasbeen performed before or after the friction stir welding according tothe present invention; and a graph showing hardness distribution of aconventional joined product on which only the friction stir welding hasbeen performed without performing the reversion treatment, the graphsbeing related to the cross sectional view of the joined Al alloymembers;

FIG. 3 is a view for explaining one example of a process of performingthe operation of reversion treatment and the operation of friction stirwelding continuously in order on the same line; and

FIG. 4 is a view for explaining one example of a process of performingthe operation of friction stir welding and the operation of reversiontreatment continuously in order on the same line.

DETAILED DESCRIPTION OF THE INVENTION

In the present method of joining heat-treatable aluminum alloy members(heat-treatable Al alloy members) by friction stir welding, theheat-treatable Al alloy members are made of known heat-treatablealuminum alloys whose hardness can be increased by an age-hardeningtreatment such as those specified in The International DesignationSystem for Wrought Aluminum and Wrought Aluminum Alloys and including a6000 series aluminum alloy (Al—Mg—Si series), a 2000 series aluminumalloy (Al—Cu—Mg series), and a 7000 series aluminum alloy (Al—Zn—Mgseries). More specifically described, examples of the 6000 seriesaluminum alloy include an AA 6061 alloy, an AA 6063 alloy or the likewith a small Si content and an AA 6016 alloy, an AA 6111 alloy or thelike with a large Si content. Examples of the 2000 aluminum alloyinclude an AA 2014 alloy, an AA 2017 alloy, an AA 2024 alloy and thelike. Examples of the 7000 series aluminum alloy include an AA 7075alloy, an AA 7003 alloy and the like.

The heat-treatable Al alloy members may have any shape such as aplate-like shape, a pipe-like shape or a bar-like shape produced by aknown method such as rolling, extrusion or casting, as long as endportions of respective heat-treatable Al alloy members at which themembers are to be joined together can be butted to each other.Generally, the plate-like members or the extruded members are suitablyused.

In particular, a plate-like rolled member is produced as follows, forinstance: Initially, an aluminum alloy having a predetermined chemicalcomposition is cast into an ingot according to an ordinary direct chillcasting method. After the obtained cast ingot has been subjected to ahomogenization heat treatment, hot rolling is performed, therebyproducing a hot-rolled plate. Alternatively, a molten metal of analuminum alloy having a predetermined chemical composition is formeddirectly into a continuous cast plate according to a continuous castingmethod (strip casting method). The hot-rolled plate or the continuouscast plate is subsequently subjected to cold rolling, so that a planarplate member with a predetermined thickness is produced. Before orduring the cold rolling, an intermediate annealing treatment may beperformed as needed.

A plurality of heat-treatable Al alloy members each as the plate member,the extruded member or the like described above are joined together byfriction stir welding. In the present invention, a T4 treatment isinitially performed on those heat-treatable Al alloy members. In otherwords, the heat-treatable Al alloy members are subjected to a solutionheat treatment and subsequent quenching, and then naturally aged, sothat the heat-treatable aluminum alloy members with T4 temper areobtained. Particularly when the 6000 series aluminum alloy is used,pre-aging at a temperature of 40-120° C. for a time period within 24hours may be performed as needed after the quenching, for the purpose ofimparting bake hardenability to the heat-treatable Al alloy members.

In the present method, the thus obtained T4 temper heat-treatable Alalloy members are joined by friction stir welding before or afterperforming a reversion treatment. There will be first explained a firstarrangement in which the heat-treatable Al alloy members are joinedafter the reversion treatment has been performed.

The reversion treatment is a treatment for temporality extinguishing theGP zones (clusters) formed in the metal structure of the T4 temperheat-treatable Al alloy members as a result of natural aging. In thepresent invention, the reversion treatment is performed by carrying outa heat treatment in which the T4 temper heat-treatable Al alloy membersare heated at an elevated temperature of 150-300° C. and held at thetemperature for a time period of not longer than 300 seconds, preferablynot longer than 60 seconds. The lower limit of the holding time is notparticularly limited. The temperature may be lowered immediately afterreaching the desired level.

In the reversion treatment mentioned above, where the heat treatmenttemperature (the reversion treatment temperature) is lower than 150° C.,the GP zones (clusters) are not sufficiently extinguished, namely, thereversion is insufficient, so that the strength or hardness of the basematerial cannot be effectively lowered. On the contrary, the heattreating temperature exceeding 350° C. causes overaging, leading tocoarsening of precipitates formed in the metal structure. In this case,the reversion structure is not obtained. Further, if the holding time atthe heat treatment temperature is excessively long, age hardening may becaused after the reversion or softening may be caused due to averagingalthough the metal structure is reverted. In this case, the intendedreversion structure is not obtained.

Preferably, the heat treatment temperature (the reversion treatmenttemperature) is precisely set depending upon the kind of the Al alloyfor the heat-treatable Al members to be joined. For instance, where theheat-treatable Al alloy members are made of the 6000 series aluminumalloy, the heat treatment temperature is preferably held in a range of200-350° C., more preferably in a range of 200-300° C. Where theheat-treatable Al alloy members is made of the 2000 series aluminumalloy, the heat treatment temperature is preferably held in a range of150-300° C., more preferably in a range of 180-300° C. Where theheat-treatable Al alloy members is made of the 7000 series aluminumalloy, the heat treatment temperature is preferably held in a range of150-250° C., more preferably in a range of 170-250° C.

In raising the temperature for heating the heat-treatable Al alloymembers to an intended level and lowering the temperature from thatlevel, the temperature raising rate and the temperature lowering rateare not particularly limited, but preferably set to be not less than 20°C./second, more preferably, in a range from 4° C./second to 50°C./second, for effectively establishing the reversion explained above.

As a result of the predetermined heat treatment performed as describedabove, the GP zones (clusters) formed in the metal structure of theheat-treatable Al alloy members with T4 temper is temporarilyextinguished, so that the Al alloy members have the reversion structure.Accordingly, the heat-treatable Al alloy members placed in the reversionstate after the reversion treatment have lowered strength or hardness asa whole.

In the method according to the first arrangement, the subsequentfriction stir welding process is performed on the heat-treatable Alalloy members whose strength or hardness has been lowered by thereversion treatment. However, in the heat-treatable Al alloy members inthe reversion state whose strength or hardness has been lowered by thereversion treatment, the GP zones (clusters) will be re-formed in themetal structure due to natural aging, so that the strength or hardnesswill be increased. In view of this, the joining operation by frictionstir welding is preferably carried out while the heat-treatable Al alloymembers are kept in the reversion state in which the strength orhardness of the Al alloy members is not excessively increased due tonatural aging, in other words, before the GP zones are substantiallyre-formed. More specifically explained, the joining operation byfriction stir welding is preferably carried out before an increase intensile strength of the heat-treatable Al alloy members, namely, adifference between tensile strength of the heat-treatable Al alloymembers upon joining by friction stir welding and tensile strengththereof after the reversion treatment exceeds 10 MPa, preferably 5 MPa.This is because the strength or hardness of the base materialundesirably becomes higher than that of the heat-affected zones afterthe friction stir welding if the increase in the tensile strength due tothe age hardening exceeds 10 MPa. In this case, it is difficult toobtain the intended strength relationship. Here, “tensile strength” isone measured in accordance with “Standard Test Methods of TensionTesting Wrought and Cast Aluminum and Magnesium Alloy Products”specified in ASTM B 557-94.

In the light of the above, where the heat-treatable Al alloy members arestored at 20° C., it is preferable to carry out the friction stirwelding process within about 60 days after the reversion treatment forthe heat-treatable Al alloy members formed of the 6000 series aluminumalloy and within about 20 days after the reversion treatment for theheat-treatable Al alloy members formed of the 2000 series aluminum alloyor the 7000 series aluminum alloy. The higher the storage temperature,the faster the age hardening proceeds. Accordingly, where theheat-treatable Al alloy members formed of the AA 6000 series aluminumalloy are stored at 40° C., it is preferable to carry out the frictionstir welding process within one week or two weeks after the reversiontreatment. Namely, it is important to carry out the friction stirwelding before the tensile strength becomes excessively large, in otherwords, before the effect of the reversion treatment is not lost.

For forming the intended joined product by joining together theheat-treatable Al alloy members in the reversion state by friction stirwelding, a known method may be employed. For instance, initially, twoheat-treatable Al alloy members 10 a, 10 b as show in FIG. 1 (each inthe form of a plate in this arrangement) are butted to each other, andthese two Al alloy members are prevented, in a known manner, from movingrelative to each other in the longitudinal direction (the weldingdirection) and the widthwise direction thereof. Subsequently, a rotarytool 12 is rotated about its axis at a high speed together with a pin14, and the rotary tool 12 and the pin 14 are moved relative to a buttedportion 16 of the two heat-treatable Al alloy members 10 a, 10 b alongthe same 16 in a direction perpendicular to the sheet plane of FIG. 1with the pin 14 inserted in the butted portion 16, whereby the twoheat-treatable Al alloy members 10 a, 10 b are joined together byfriction stir welding at the butted portion 16. In this connection, therotary tool 12 and the pin 14 may be moved along the butted portion 16or the two heat-treatable Al alloy members 10 a, 10 b which areprevented from moving relative to each other (i.e., which are bound toeach other) may be moved.

After the friction stir welding operation, there is formed, at thebutted portion 16 at which the two heat-treatable Al alloy members 10 a,10 b are butted to each other, a stir zone 18 as shown in the crosssectional view of the upper part of FIG. 2 bridging the two Al alloymembers 10 a, 10 b and extending continuously in the longitudinaldirection (i.e., in a direction perpendicular to the sheet plane of FIG.2). Further, in the vicinity of and adjacent to the stir zone 18, thereexist heat-affected zones 20 (HAZ portions) affected by the heat in thefriction stir welding operation.

In a conventional arrangement, the heat-affected zones 20 have thelowest hardness as shown in the lower graph of FIG. 2. In contrast,since the reversion treatment is performed prior to the friction stirwelding operation in the present arrangement, the stir zone 18 has astructure similar to that when subjected to a solution heat treatmentand its hardness is larger than those of base material portions 22 andthe heat-affected zones 20, as shown in the upper graph of FIG. 2. Morespecifically explained, the temperature of the stir zone 18 reaches 450°C. or higher, so that alloy components such as Si, Mg, Cu and the likethat constitute the cluster are solubilized. Accordingly, the rate ofhardening of the stir zone 18 due to natural aging after the welding isefficiently increased as compared with those of the base materialportions 22 and the heat-affected zones 20, whereby the stir zone 18 hasthe highest hardness. The heat-affected zones 20 do not suffer fromsoftening even subjected to the heat and have the reversion structurewhich has undergone slight age-hardening. The hardness of theheat-affected zones 20 is lower than that of the stir zone 18 and isequal to or higher than that of the base material portions 22. The basematerial portions 22 which are not influenced by the heat of thefriction stir welding maintain the reversion structure and have thelowest hardness. Consequently, the distribution of the strength of thejoined product in a direction perpendicular to the welding directionassumes a trapezoidal shape shown in the upper graph of FIG. 2 in whichthe strength increases from the base material portions 22 located on theopposite sides of the stir zone 18 toward the central stir zone 18 (thebutted portion 16).

In contrast, in a case where the reversion treatment is not carried out,the stir zone 18 has, after the friction stir welding, a structuresimilar to that when subjected to the solution heat treatment and itshardness is generally equal to or slightly lower than that of the basematerial portions 22 with T4 temper, as shown in the lower graph of FIG.2. In the meantime, the heat-affected zones 20 have the metal structurein the reversion state and the hardness thereof is the lowest.Therefore, upon application of a stress to such a joined product, thestress concentrates on the heat-affected zones 20 having the lowesthardness, so that the heat-affected zones 20 tend to easily deformfaster than the other portions of the joined product, undesirablycausing fracture or breakage.

The study made by the inventors of the present invention revealed thatthe hardness distribution with the trapezoidal shape shown in the uppergraph of FIG. 2 is not obtained even if the heat-treatable Al alloymembers having the reversion structure as a result of implementation ofthe reversion treatment are joined by fusion welding such as the TIGwelding, the MIG welding and the like, and it was observed that theheat-affected zones had the lowest hardness. Moreover, because thewelded region and a part of the heat-affected zones adjacent to thewelded region are once melted, the cast structure is altered ordeteriorated, resulting in lowered ductility.

In the first arrangement explained above wherein the friction stirwelding operation is performed on the heat-treatable Al alloy membersplaced in the reversion state, the relationship in strength among thestir zone 18, the heat-affected zones 20 and the base material portions22 can be optimized, thereby advantageously providing the joined productin which the base material portions 22 have the lowest hardness.Therefore, even upon application of a stress to the joined product, itis possible to effectively prevent the stress from concentrating on theheat-affected zones 20, whereby the stress is distributed at theentirety of the base material portions 22. Thus, the overall deformationamount of the joined product is advantageously increased, making itpossible to perform press forming with a high degree of freedom in theconfiguration of the press product to be obtained and with a largerdeformation amount. In other words, the press formability of the joinedproduct can be considerably effectively enhanced.

The thus obtained joined product having excellent press formability isadvantageously used as a blank for press forming operation, forinstance, and used as shell members of vessels, vehicles, aircrafts andthe like, floors, building materials, heat exchangers, antennas, vehiclecomponents, bridges and the like, after the joined product has beensubjected to the press forming operation.

The heating system in the reversion treatment performed prior to thefriction stir welding is not particularly limited, but any heating meansknown in the art may be employed. For instance, where the step ofperforming the reversion treatment and the step of performing thefriction stir welding operation are carried out discontinuously, e.g.,where any one of the steps is carried out separately from a line towhich the other of the steps belongs, a heating means for performing theheat treatment is preferably selected from a salt bath, an oil bath, anair-heating furnace, an iron, infrared heating and induction heating,from the viewpoint of equipment and cost.

In the present arrangement, the reversion treatment and the frictionstir welding operation may be carried out continuously on the same line.Where the reversion treatment and the friction stir welding operationare carried out on the same line, for instance, the aforementionedreversion treatment (heat treatment) is performed sequentially along thebutted portion 16 of the two heat-treatable Al alloy members 10 a, 10 bat which the two members 10 a, 10 b are welded, over a sufficientlylarge area ranging from the butted portion 16 to the base materialportions 22 in a widthwise direction of the two members 10 a, 10 b (thatis perpendicular of the welding direction), preferably, over an entirearea in the widthwise direction, while the friction stir weldingoperation is performed sequentially on the butted portion 16 which hasbeen subjected to the reversion treatment, as shown in FIG. 3. Where thereversion treatment and the friction stir welding are continuouslycarried out on the same line, it is preferable to employ, as the heatingmeans, infrared heating, induction heating, laser heating, gas flameheating, or the like. In this instance, the reversion treatment (heattreatment) needs to be performed not only on the butted portion 16 andits vicinity, but also over a large area in the widthwise direction ofthe heat-treatable Al alloy members, for assuring a large deformationamount by permitting the stress to be applied upon working operation tobe effectively distributed over the base material portions.

In the illustrated first arrangement, the planar heat-treatable Al alloymembers 10 a, 10 b having the same thickness and formed of the same kindof heat-treatable Al alloy are joined together. The heat-treatable Alalloy members having mutually different thickness or formed of differentkinds of Al alloy may be joined together. Where the heat-treatable Alalloy members having mutually different thickness are joined, thereversion treatment may be performed on both of a thin Al alloy memberand a thick Al alloy member or only on the thin Al alloy member. Namely,the aforementioned T4 treatment, reversion treatment and friction stirwelding operation may be sequentially performed such that a portionwhose strength or hardness becomes the lowest after welding correspondsto a base material portion of one of the two heat-treatable Al alloymembers to be joined.

Next, there will be explained a second arrangement in which theheat-treatable Al alloy members are joined before the reversiontreatment is performed. Described more specifically, the T4-treatedheat-treatable Al alloy members are joined together by friction stirwelding in a known manner, thereby providing a joined product. Becausethe manner in which the T4-treated heat-treatable Al alloy members arejoined by friction stir welding has been explained above with respect tothe illustrated first arrangement referring to FIGS. 1 and 2, a detailedexplanation of which is not given here. As in the illustrated firstarrangement, after the friction stir welding operation, there is formed,at the butted portion 16 at which the two heat-treatable Al alloymembers 10 a, 10 b are butted to each other, a stir zone 18 as shown inthe cross sectional view of the upper part of FIG. 2 bridging the two Alalloy members 10 a, 10 b and extending continuously in the longitudinaldirection (i.e., in a direction perpendicular to the sheet plane of FIG.2). In this second arrangement, as a result of the friction stirwelding, the temperature of the stir zone 18 reaches 450° C. or higher,so that substances such as Si, Mg, Cu, Zn and the like that contributeto the strength and alloy components that form the GP zones (clusters)are solubilized. Accordingly, the stir zone 18 has a structure similarto that when subjected to a solution heat treatment in which the GPzones (clusters) are not present, i.e., a so-called solutionizedstructure, and the hardness of the stir zone 18 temporarily becomes thelowest. In the vicinity of and adjacent to the stir zone 18, there existthe heat-affected zones 20 (i.e., HAZ) affected by the heat of thefriction stir welding so as to be placed in a reversion state. Thehardness of the heat-affected zones 20 is lower than that of the basematerial. In the meantime, the base material portions 22 which are notinfluenced by the heat of the friction stir welding maintains the T4temper and has the highest hardness.

In the second arrangement, with the metal structure immediately afterthe friction stir welding substantially maintained (i.e., thesolutionized structure of the stir zone 18, the reversion structure ofthe heat-affected zones and the T4 temper state of the base materialportions 22), the reversion treatment is subsequently carried out. Asexplained above with respect to the illustrated first arrangement, thereversion treatment is for temporality extinguishing the GP zones(clusters) formed in the metal structure of the T4-treatedheat-treatable Al alloy members as a result of natural aging. Thereversion treatment is performed by carrying out a heat treatment inwhich the joined product obtained by joining the T4-treatedheat-treatable Al alloy members by friction stir welding are heated atan elevated temperature of 150-300° C. and held at the temperature for atime period of not longer than 300 seconds, preferably not longer than60 seconds. The lower limit of the holding time is not particularlylimited. The temperature may be lowered immediately after reaching thedesired level.

The heat treatment temperature (the reversion treatment temperature)lower than 150° C. or higher than 350° C. and the excessively longholding time at the heat treatment temperature will cause thedisadvantages described above with respect to the illustrated firstarrangement. As in the illustrated first arrangement, the heat treatmenttemperature (the reversion treatment temperature) may be precisely setdepending upon the kind of the heat-treatable aluminum alloy used forproviding the heat-treatable Al alloy members. Further, in raising thetemperature for heat treating the joined product obtained to an intendedlevel and lowering the temperature from that level, the temperatureraising rate and the temperature lowering rate are not particularlylimited, but preferably set to be not less than 2° C./second, morepreferably, in a range from 4° C./second to 50° C./second, foreffectively establishing the reversion explained above, as in theillustrated first arrangement.

In this second arrangement, the reversion treatment explained above iscarried out after the friction stir welding operation has been carriedout. In this respect, the metal structure of the joined product obtainedby the friction stir welding changes with a lapse of time, so that thestrength or hardness of the joined product changes. Therefore, it isdesirable to perform the reversion treatment before the metal structurelargely changes, in other words, before the GP zones (clusters) areformed in the metal structure of the stir zone 18 having thesolutionized structure in which the rate of natural aging is high. Morespecifically described, it is preferable to carry out the reversiontreatment before a time period during which the joined product is heldat room temperature after the friction stir welding exceeds 24 hours,preferably 6 hours. If the time period between the completion of thefriction stir welding and the initiation of the reversion treatment istoo long, the GP zones (clusters) are undesirably formed in the metalstructures of the stir zone 18 and the heat-affected zones 20 due tonatural aging. Even if the reversion treatment is performed on thejoined product having such a metal structure, it is not possible toattain the hardness distribution of the trapezoidal shape shown in theupper graph of FIG. 2 explained above with respect to the illustratedfirst arrangement and the strength or hardness of the stir zone 18 andthe strength or hardness of the heat-affected zones 20 become equal toor slightly smaller than that of the base material. Consequently,fracture is likely to occur at the stir zone 18 and the heat-affectedzones 20, especially at the heat-affected zones 20.

The heating system in the reversion treatment to be performed after thefriction stir welding is not particularly limited, but any heating meansknown in the art may be employed. For instance, where the step ofperforming the friction stir welding operation and the step ofperforming the reversion treatment are carried out discontinuously,e.g., where any one of the steps is carried out separately from a lineto which the other of the steps belongs, a heating means for performingthe heat treatment is preferably selected from a salt bath, an oil bath,an air-heating furnace, an iron, infrared heating and induction heating,from the viewpoint of equipment and cost.

In the present arrangement, the friction stir welding operation and thereversion treatment may be carried out continuously on the same line.Where the friction stir welding operation and the reversion treatmentare carried out on the same line, the friction stir welding operation isperformed sequentially along the butted portion 16 at which theheat-treatable Al alloy members 10 a, 10 b are to be joined together,while the reversion treatment is performed sequentially on the joinedproduct over a sufficiently large area ranging from the stir zone 18,the heat-affected zones 20, to the base material portions 22 in awidthwise direction of the joined product (that is perpendicular of thewelding direction), preferably, over an entire area in the widthwisedirection, as shown in FIG. 4. Where the friction stir welding operationand the reversion treatment are continuously carried out on the sameline, it is preferable to employ, as the heating means, infraredheating, induction heating, laser heating, gas flame heating or thelike. In this instance, the reversion treatment (the heat treatment)needs to be performed not only on the stir zone 18 and its vicinity, butalso over a large area in the widthwise direction of the joined product,for assuring a large deformation amount by permitting the stress to beapplied upon working operation to be effectively distributed over thebase material portions.

As a consequence of the suitable heat treatment (reversion treatment)performed on the joined product as mentioned above, the GP zones(clusters) formed in the metal structure of the base material portions22 are temporarily extinguished, so that the base material portions 22have the reversion structure and the strength or hardness of the basematerial portions 22 is advantageously lowered. In the meantime, thestir zone 18 maintains the solutionized structure in which no GP zones(clusters) exist while the heat-affected zones 20 have the reversionstructure which has undergone slight age-hardening. Where the joinedproduct in such a metal structure state is naturally aged at roomtemperature, the hardness becomes the highest at the stir zone 18 owingto a high rate of age-hardening. The hardness of the heat-affected zones20 is lower than that of the stir zone 18 and equal to or higher thanthat of the base material portions 22. The hardness of the base materialportions 22 is lower than those of the stir zone 18 and theheat-affected zones 20. Consequently, the joined product according tothe second arrangement has a hardness distribution in a directionperpendicular to the welding direction assumes a trapezoidal shape inwhich the hardness increases from the base material portions 22 locatedon the opposite sides of the stir zone 18 toward the central stir zone18, as shown in the upper graph of FIG. 2.

In the second arrangement explained above wherein the reversiontreatment is performed on the joined product obtained by friction stirwelding the T4 temper heat-treatable Al alloy members, the relationshipin strength among the stir zone 18, the heat-affected zones 20 and thebase material portions 22 can be optimized, thereby advantageouslyproviding the joined product in which the base material portions 22 havethe lowest hardness. Therefore, even upon application of a stress to thejoined product, it is possible to effectively prevent the stress fromconcentrating on the heat-affected zones 20, whereby the stress isdistributed on the entirety of the base material portions 22. Thus, theoverall deformation amount of the joined product is advantageouslyincreased, making it possible to perform press forming with a highdegree of freedom in the configuration of the press product to beobtained and with a larger deformation amount. In other words, the pressformability of the joined product can be considerably effectivelyenhanced. Since the thus obtained joined product has excellent pressformability, it is advantageously used as a blank in press forming forvarious applications mentioned above with respect to the illustratedfirst arrangement.

Where the aforementioned reversion treatment is not carried out, thestir zone 18 has a structure similar to that when subjected to thesolution heat treatment and its hardness after natural aging isgenerally equal to or slightly lower than that of the base materialportions 22 with T4 temper, as shown in the lower graph of FIG. 2. Inthe meantime, the heat-affected zones 20 have the metal structure in thereversion state and the hardness thereof is the lowest. Therefore, uponapplication of a stress to such a joined product, the stressconcentrates on the heat-affected zones 20 having the lowest hardness,so that the heat-affected zones 20 tend to easily deform faster than theother portions of the joined product, undesirably causing fracture orbreakage. The study made by the inventors of the present inventionrevealed the following: In a case even where the aforementionedreversion treatment is performed on the T4 temper joined productobtained by fusion welding such as the TIG welding, the MIG welding andthe like, the welded region and the heat-affected zones located adjacentto the welded region are once molten, whereby the cast structure isaltered or deteriorated and the ductility is lowered. Therefore, it wasobserved that the hardness distribution with the trapezoidal shape shownin the upper graph of FIG. 2 was not obtained and the hardness is thelowest at the heat-affected zones.

In the illustrated second arrangement, the planar heat-treatable Alalloy members 10 a, 10 b having the same thickness and formed of thesame kind of heat-treatable Al alloy are joined together. Theheat-treatable Al alloy members having mutually different thickness orformed of different kinds of Al alloy may be joined together.

While preferred embodiments of the present invention have been describedin detail by reference to the accompanying drawings, it is to beunderstood that the present invention may be embodied with variouschanges and modifications that may occur to those skilled in the art,without departing from a scope of the invention defined in attachedclaims.

EXAMPLES

To further clarify the present invention, some examples of the presentinvention will be described. It is to be understood that the presentinvention is not limited to the details of these examples and theforegoing description.

Example 1

Ten aluminum alloys A-J having respective chemical compositions shown inthe following TABLE 1 were cast into ingots according to a DC castingmethod in an ordinary manner. The obtained cast ingots were thensubjected to homogenization, hot rolling and cold rolling, therebyproviding aluminum alloy plate members each having thickness of 1.0 mm.Subsequently, the aluminum alloy plate members were subjected to asolution heat treatment and quenching into water, and then naturallyaged at room temperature for ten days. Thus, there were obtained T4temper members.

TABLE 1 Composition (mass %) Alloy Si Fe Cu Mn Mg Cr Zn Ti Al Series ofalloy A 0.6 0.32 4.5 0.8 0.42 0.02 0.09 0.04 balance 2000 B 0.5 0.28 4.30.6 0.6 0.02 0.07 0.03 balance 2000 C 0.12 0.18 4.5 0.6 1.5 0.01 0.020.02 balance 2000 D 0.42 0.15 0.03 0.06 0.5 0.02 0.03 0.03 balance 6000E 0.7 0.13 0.32 0.12 1.1 0.18 0.02 0.02 balance 6000 F 1.0 0.16 0.010.07 0.5 0.01 0.01 0.02 balance 6000 G 1.0 0.12 0.01 0.09 0.5 0.02 0.230.02 balance 6000 H 1.0 0.15 0.7 0.09 0.4 0.03 0.01 0.03 balance 6000 I0.11 0.17 0.16 0.25 1.3 0.09 4.6 0.02 balance 7000 J 0.13 0.18 1.6 0.032.7 0.22 5.6 0.03 balance 7000

Subsequently, the thus obtained T4 temper members were subjected to aheat treatment (reversion treatment) under respective conditionsindicated in the following TABLE 2 and stored at 20° C. for three days.Two aluminum alloy members formed of each of the alloys A-J andsubjected to the reversion treatment were butt-joined by a friction stirwelding (FSW) process with their widthwise end portions butted to eachother, thereby producing test members 1-10 each as a joined product. Itwas separately confirmed that, in any of the aluminum alloy members, anincrease in tensile strength during a time period after the reversiontreatment and before the welding operation was not greater than 10 MPa.In the friction stir welding operation, a rotary tool made of steel wasrotated at 1000 rpm and moved horizontally at a speed of 400 mm/minute.The rotary tool was provided at its end with eight grooves each having adepth of 1 mm for the purpose of chipping or scraping.

Thereafter, the test members 1-10 obtained by the friction stir weldingwere stored at 20° C. for seven days and then subjected to a Vickershardness test, a tensile test and a formability test explained below.The results of the tests are indicated also in TABLE 2.

Vickers Hardness Test

A test piece was cut from each test member in a direction perpendicularto a welding direction of the test member. Then, the test piece wasembedded in a resin and was ground at its cross section in a directionperpendicular to the welding direction. The hardness of the stir zone,heat-affected zones and base material portions was measured using aVickers hardness tester with a load of 1 kgf according to ASTM E 92.

Tensile Test

A rectangular specimen defined in ASTM B 557-94 was cut from each testmember such that the welding direction of the test member is orthogonalto a tensile direction in the tensile test and such that the weldedregion is located at a middle of the specimen. The tensile testconforming to ASTM B 557-94 was performed on the obtained specimen atroom temperature. For each specimen, tensile strength, yield strengthand breaking elongation were measured. Further, a position at whichfracture or breakage occurred (hereinafter may be referred to as“fractured position”) was observed for each specimen.

Formability Test

A circular specimen with a diameter of 120.0 mm was cut from each testmember such that the welded region is located at a center of thespecimen. After the circular specimen was coated on a surface thereofwith a low viscous lubricant, a bulging operation is conducted using anErichsen tester so as to measure a forming limit height until fracture.The bulging operation was conducted using a hemispherical punch with adiameter of 50 mm at a forming rate of 2 mm/second with the circularspecimen fixedly held by a die with a lock bead for preventing aperipheral portion of the specimen from flowing toward a centralportion.

TABLE 2 Vickers hardness Tensile properties Heat treatment Heat- TensileYield Forming Test Temperature Time Stir affected Base strength strengthElongation limit height member Alloy (° C.) (second) zone zones material(MPa) (MPa) (%) Fractured position (mm) 1 A 250 20 118 115 110 402 29520 base material 16.5 2 B 240 30 120 118 113 413 286 21 base material16.0 3 C 250 30 135 127 125 455 337 20 base material 15.7 4 D 220 10 5953 50 185 83 24 base material 15.2 5 E 270 20 74 71 66 235 125 22 basematerial 16.0 6 F 250 5 67 64 60 218 122 24 base material 17.0 7 G 25010 69 65 62 220 123 25 base material 17.1 8 H 270 20 76 74 70 253 135 23base material 17.9 9 I 200 30 114 112 110 355 210 16 base material 15.810 J 180 20 120 115 113 396 220 17 base material 16.2

It will be apparent from the above TABLE 2 that, in any of the testmembers 1-10, the hardness of the base material is the lowest among thestir zone, the heat-affected zones and the base material. It isrecognized from the results of the tensile test that, in any of the testmembers 1-10, fracture occurred not at the heat-affected zones, but atthe base material and that all of the test members 1-10 exhibit thebreaking elongation of 16% or larger. It is further recognized from theresults of the formability test that all of the test members 1-10 havethe forming limit height of 15.0 mm or larger. It can be understood fromthese results that the test members 1-10 on which the reversiontreatment was performed according to the present invention exhibitexcellent press formability that permits sufficient deformation in pressforming.

Comparative Example 1

For comparison, initially, ten aluminum alloys A-J having respectivechemical compositions which are the same as those in the above Example 1were cast into ingots according to a DC casting method in an ordinarymanner. The obtained cast ingots were then subjected to homogenization,hot rolling and cold rolling, thereby providing aluminum alloy platemembers each having thickness of 1.0 mm. Subsequently, the aluminumalloy plate members were subjected to a solution heat treatment andquenching into water, and then naturally aged at room temperature forten days. Thus, there were obtained T4 temper members. Withoutperforming the heat treatment (reversion treatment), two aluminum alloymembers with T4 temper formed of each of the alloys A-J were butt-joinedby the friction stir welding (FSW) process under conditions similar tothose in the above Example 1 with their widthwise end portions butted toeach other, thereby producing test members 11-20 each as a joinedproduct. Thereafter, the test members 11-20 obtained by the frictionstir welding were stored at 20° C. for seven days and then subjected tothe Vickers hardness test, the tensile test and the formability testexplained above. The results of the tests are indicated also in TABLE 3.

TABLE 3 Vickers hardness Tensile properties Heat treatment Heat- TensileYield Forming Test Temperature Time Stir affected Base strength strengthElongation limit height member Alloy (° C.) (second) zone zones material(MPa) (MPa) (%) Fractured position (mm) 11 A — — 122 108 125 382 295 10Heat-affected zones 13.0 12 B — — 125 112 128 387 280 10 Heat-affectedzones 12.7 13 C — — 130 119 138 408 339 11 Heat-affected zones 12.5 14 D— — 54 47 57 160 87 12 Heat-affected zones 12.2 15 E — — 74 68 76 228132 12 Heat-affected zones 13.3 16 F — — 72 64 72 218 128 11Heat-affected zones 14.2 17 G — — 73 64 73 217 129 11 Heat-affectedzones 14.3 18 H — — 78 72 80 250 142 12 Heat-affected zones 14.6 19 I —— 120 112 124 360 245 9 Heat-affected zones 13.5 20 J — — 123 114 124390 232 9 Heat-affected zones 13.8

It will be apparent from the results shown in the above TABLE 3 that, inany of the test members 11-20, the Vickers hardness of the heat-affectedzones is the lowest among the stir zone, the heat-affected zones and thebase material since the reversion treatment was not performed. Further,in any of the test members 11-20, fracture occurred at the heat-affectedzones. Moreover, all of the test members 11-20 have smaller values ofbreaking elongation and smaller values of forming limit height less than15.0 mm, as compared with the test members 1-10 of the above Example 1each of which was obtained by joining two aluminum alloy members formedof the same aluminum alloy.

Comparative Example 2

For comparison, initially, ten aluminum alloys A-J having respectivechemical compositions which are the same as those in the above Example 1were cast into ingots according to a DC casting method in an ordinarymanner. The obtained cast ingots were then subjected to homogenization,hot rolling and cold rolling, thereby providing aluminum alloy platemembers each having thickness of 1.0 mm. Subsequently, the aluminumalloy plate members were subjected to a solution heat treatment andquenching into water, and then naturally aged at room temperature forten days. Thus, there were obtained T4 temper members. Subsequently, thethus obtained T4 temper members were subjected to a heat treatment underrespective conditions indicated in the following TABLE 4 and stored at20° C. for three days. Two aluminum alloy members formed of each of thealloys A-J and subjected to the heat treatment were butt-joined by afriction stir welding (FSW) process under conditions similar to those inthe above Example 1 with their widthwise end portions butted to eachother, thereby producing test members 21-50 each as a joined product.Thereafter, the test members 21-50 obtained by the friction stir weldingwere stored at 20° C. for seven days and then subjected to the Vickershardness test, the tensile test and the formability test explainedabove. The results of the tests are indicated also in TABLE 4.

TABLE 4 Vickers hardness Tensile properties Heat treatment Heat- TensileYield Forming Test Temperature Time Stir affected Base strength strengthElongation limit height member Alloy (° C.) (second) zone zones material(MPa) (MPa) (%) Fractured position (mm) 21 A 120 30 119 111 123 377 29210 Heat-affected zones 12.8 22 A 380 30 97 92 91 305 253 15 Basematerial 14.7 23 A 250 1800 100 99 97 316 247 14 Base material 14.4 24 B120 30 124 110 127 386 282 10 Heat-affected zones 12.6 25 B 380 30 93 9089 297 251 15 Base material 14.3 26 B 250 1800 97 97 94 305 242 15 Basematerial 14.2 27 C 120 30 130 118 137 410 340 13 Heat-affected zones12.4 28 C 380 30 96 92 92 300 260 15 Base material 14.1 29 C 250 1800 9995 97 310 268 14 Base material 13.8 30 D 120 30 55 46 57 159 87 12Heat-affected zones 12.4 31 D 380 30 49 43 42 157 75 13 Base material13.8 32 D 250 1800 56 54 55 180 152 10 Base material 13.2 33 E 120 30 7568 76 230 130 12 Heat-affected zones 13.5 34 E 380 30 62 64 60 210 11112 Base material 14.8 35 E 250 1800 77 75 75 245 212 10 Base material14.2 36 F 120 30 73 65 72 218 129 11 Heat-affected zones 14.2 37 F 38030 58 58 56 196 112 12 Base material 13.8 38 F 250 1800 74 74 72 232 19811 Base material 14.0 39 G 120 30 74 65 73 217 128 11 Heat-affectedzones 14.1 40 G 380 30 59 59 55 197 111 12 Base material 13.8 41 G 2501800 74 75 71 231 197 11 Base material 13.9 42 H 120 30 78 70 79 242 14312 Heat-affected zones 14.4 43 H 380 30 65 64 62 215 118 11 Basematerial 14.0 44 H 250 1800 83 81 80 258 223 10 Base material 13.7 45 I120 30 119 110 124 360 243 9 Heat-affected zones 13.6 46 I 380 30 95 9291 315 195 12 Base material 14.2 47 I 250 1800 71 72 70 223 132 14 Basematerial 14.0 48 J 120 30 124 114 123 390 233 9 Heat-affected zones 13.949 J 380 30 82 83 80 275 157 11 Base material 14.6 50 J 250 1800 76 7677 252 125 12 Base material 14.2

It will be apparent from the results indicated in the above TABLE 4that, in each of the test members 21, 24, 27, 30, 33, 36, 39, 42, 45 and48, the metal structure of the T4-treated aluminum alloy member was notreverted due to the low heat treatment temperature, the Vickers hardnessof the base material was higher than that of the heat-affected zones,and fracture occurred at the heat-affected zones. Further, these testmembers have low values of breaking elongation and forming limit height.In each of the test members 22, 25, 28, 31, 34, 37, 40, 43, 46 and 49,softening by overaging was caused due to the high heat treatmenttemperature, so that the intended reversion structure was not obtained.Accordingly, these test members have low values of breaking elongationand forming limit height though fracture occurred at the base materialin the tensile test. In each of the test members 23, 26, 29, 32, 38, 41,44, 47 and 50, softening by overaging was caused due to the excessivelylong heat treatment time, so that the intended reversion structure wasnot obtained. Accordingly, these test members have low values ofbreaking elongation and forming limit height though fracture occurred atthe base material in the tensile test.

Comparative Example 3

For comparison, initially, ten aluminum alloys A-J having respectivechemical compositions which are the same as those in the above Example 1were cast into ingots according to a DC casting method in an ordinarymanner. The obtained cast ingots were then subjected to homogenization,hot rolling and cold rolling, thereby providing aluminum alloy platemembers each having thickness of 1.0 mm. Subsequently, the aluminumalloy plate members were subjected to a solution heat treatment andquenching into water, and then naturally aged at room temperature forten days. Thus, there were obtained T4 temper members. Subsequently, thethus obtained T4 temper members were subjected to a heat treatment(reversion treatment) under respective conditions indicated in thefollowing TABLE 5 and stored at 40° C. for 30 days. Two aluminum alloymembers formed of each of the alloys A-J and subjected to the heattreatment were butt-joined by the friction stir welding (FSW) processunder conditions similar to those in the above Example 1 with theirwidthwise end portions butted to each other, thereby producing testmembers 51-60 each as a joined product. It is noted that, in any of thealuminum alloy members, an increase in tensile strength during a timeperiod after the reversion treatment and before the friction stirwelding exceeded 10 MPa. Thereafter, the test members 51-60 obtained bythe friction stir welding were stored at 20° C. for seven days and thensubjected to the Vickers hardness test, the tensile test and theformability test explained above. The results of the tests are indicatedalso in TABLE 5.

TABLE 5 Vickers hardness Tensile properties Heat treatment Heat- TensileYield Forming Test Temperature Time Stir affected Base strength strengthElongation limit height member Alloy (° C.) (second) zone zones material(MPa) (MPa) (%) Fractured position (mm) 51 A 250 20 120 112 142 380 3129 Heat-affected zones 12.5 52 B 240 30 121 115 135 384 294 9Heat-affected zones 12.4 53 C 250 30 135 122 148 402 351 10Heat-affected zones 12.1 54 D 220 10 60 50 63 155 99 12 Heat-affectedzones 11.9 55 E 270 20 74 69 80 219 144 11 Heat-affected zones 12.9 56 F250 5 69 62 79 206 141 10 Heat-affected zones 13.7 57 G 250 10 70 62 80207 140 10 Heat-affected zones 13.6 58 H 270 20 77 70 85 246 154 11Heat-affected zones 14.2 59 I 200 30 110 114 132 352 258 9 Heat-affectedzones 13.4 60 J 180 20 121 112 128 386 239 8 Heat-affected zones 13.2

It is understood from the results shown in the above TABLE 5 that, inany of the test members 51-60, the friction stir welding operation wasperformed on the aluminum alloy members in a state in whichage-hardening occurred after the reversion state, so that the Vickershardness of the heat-affected zones was the lowest among the stir zone,the heat-affected zones and the base material. Accordingly, fractureoccurred at the heat-affected zones. Moreover, all of the test members51-60 have smaller values of elongation and smaller values of forminglimit height less than 15.0 mm, as compared with the test members 1-10in the above Example 1 each of which was obtained by joining twoaluminum alloy members formed of the same aluminum alloy.

Example 2

Ten aluminum alloys K-T having respective chemical compositions shown inthe following TABLE 6 were cast into ingots according to a DC castingmethod in an ordinary manner. The obtained cast ingots were thensubjected to homogenization, hot rolling and cold rolling, therebyproviding aluminum alloy plate members each having thickness of 1.0 mm.Subsequently, the aluminum alloy plate members were subjected to asolution heat treatment and quenching into water, and then naturallyaged at room temperature for seven days. Thus, there were obtained T4temper members.

TABLE 6 Composition (mass %) Alloy Si Fe Cu Mn Mg Cr Zn Ti Al Series ofalloy K 0.6 0.28 4.4 0.8 0.44 0.03 0.07 0.03 balance 2000 L 0.5 0.29 4.30.6 0.6 0.04 0.08 0.02 balance 2000 M 0.14 0.20 4.5 0.7 1.5 0.02 0.030.02 balance 2000 N 0.44 0.18 0.04 0.06 0.5 0.03 0.04 0.02 balance 6000O 0.7 0.15 0.34 0.15 1.1 0.22 0.02 0.03 balance 6000 P 1.0 0.14 0.010.09 0.5 0.02 0.01 0.02 balance 6000 Q 1.0 0.15 0.02 0.07 0.5 0.01 0.210.02 balance 6000 R 1.0 0.18 0.7 0.07 0.4 0.02 0.01 0.02 balance 6000 S0.14 0.19 0.14 0.24 1.3 0.08 4.6 0.03 balance 7000 T 0.16 0.19 1.6 0.022.7 0.21 5.7 0.04 balance 7000

Two T4 temper members formed of each of the alloys K-T were butt-joinedby the friction stir welding (FSW) process with their widthwise endportions butted to each other, thereby producing a joined product. Thefriction stir welding operation was carried out using a rotary toolsimilar to that used in the above Example 1 under the same conditions asthe above Example 1. The obtained joined products were subjected to aheat treatment (reversion treatment) under respective conditionsindicated in the following TABLE 7 within one hour after the frictionstir welding.

The joined products 61-70 which had been subjected to the heat treatment(reversion treatment) were stored at 20° C. for seven days. Then, theVickers hardness test, the tensile test, and the formability testdescribed above were performed on the joined products 61-70. The resultsof the tests are also indicated in the following TABLE 7.

TABLE 7 Vickers hardness Tensile properties Heat treatment Heat- TensileYield Forming Test Temperature Time Stir affected Base strength strengthElongation limit height member Alloy (° C.) (second) zone zones material(MPa) (MPa) (%) Fractured position (mm) 61 K 250 10 119 116 109 401 29619 base material 16.3 62 L 240 20 118 116 111 410 283 21 base material15.9 63 M 250 20 137 128 124 453 336 19 base material 15.8 64 N 220 2060 54 50 186 83 23 base material 15.2 65 O 270 10 75 70 67 234 124 23base material 16.1 66 P 250 10 68 65 61 219 123 24 base material 17.1 67Q 250 5 69 64 60 218 123 25 base material 17.0 68 R 270 20 77 73 69 252136 24 base material 17.8 69 S 200 20 115 113 110 355 209 17 basematerial 15.7 70 T 180 30 119 114 111 394 221 18 base material 16.3

It will be apparent from the results indicated in the above TABLE 7that, in any of the test members 61-70, the hardness of the basematerial is the lowest among the stir zone, the heat-affected zones andthe base material. It is recognized from the results of the tensile testthat, in any of the test members 61-70, fracture occurred not at theheat-affected zones, but at the base material and that all of the testmembers 61-70 exhibit the breaking elongation of 16% or larger. It isfurther recognized from the results of the formability test that all ofthe test members 61-70 have the forming limit height of 15.0 mm orlarger. It can be understood from these results that the test members61-70 on which the reversion treatment was performed according to thepresent invention exhibit excellent press formability that permitssufficient deformation in press forming.

Comparative Example 4

For comparison, initially, ten aluminum alloys K-T having respectivechemical compositions which are the same as those in the above Example 2were processed in the same manner as in the above Example 2 to providealuminum alloy plate members each having thickness of 1.0 mm.Subsequently, the aluminum alloy plate members were subjected to asolution heat treatment and quenching into water, and then naturallyaged at room temperature for seven days. Thus, there were obtained T4temper members. Two aluminum alloy members with T4 temper formed of eachof the alloys K-T were butt-joined by the friction stir welding (FSW)process under conditions similar to those in the above Example 2 withtheir widthwise end portions butted to each other, thereby producingtest members 71-80 each as a joined product. After the friction stirwelding, the test members 71-80 were stored at 20° C. for seven dayswithout performing a heat treatment (reversion treatment), and thensubjected to the Vickers hardness test, the tensile test and theformability test explained above. The results of the tests are indicatedalso in TABLE 8.

TABLE 8 Vickers hardness Tensile properties Heat treatment Heat- TensileYield Forming Test Temperature Time Stir affected Base strength strengthElongation limit height member Alloy (° C.) (second) zone zones material(MPa) (MPa) (%) Fractured position (mm) 71 K — — 121 106 127 381 297 11Heat-affected zones 12.9 72 L — — 124 110 130 386 281 10 Heat-affectedzones 12.6 73 M — — 129 118 138 408 338 12 Heat-affected zones 12.4 74 N— — 55 47 59 161 89 11 Heat-affected zones 12.0 75 O — — 73 69 77 227134 11 Heat-affected zones 13.2 76 P — — 73 65 73 217 129 10Heat-affected zones 14.2 77 Q — — 74 64 75 216 130 11 Heat-affectedzones 14.4 78 R — — 79 71 82 252 144 12 Heat-affected zones 14.7 79 S —— 121 113 124 361 246 10 Heat-affected zones 13.4 80 T — — 125 116 126389 231 9 Heat-affected zones 13.9

It will be apparent from the results shown in the above TABLE 8 that, inany of the test members 71-80, the Vickers hardness of the heat-affectedzones is the lowest among the stir zone, the heat-affected zones and thebase material since the reversion treatment was not performed after thefriction stir welding. Further, in any of the test members 71-80,fracture occurred at the heat-affected zones. Moreover, all of the testmembers 71-80 have smaller values of elongation and smaller values offorming limit height less than 15.0 mm, as compared with the testmembers 61-70 of the above Example 2 each of which was obtained byjoining two aluminum alloy members formed of the same aluminum alloy.

Comparative Example 5

For comparison, initially, ten aluminum alloys K-T having respectivechemical compositions which are the same as those in the above Example 2were processed in the same manner as in the above Example 2 to providealuminum alloy plate members each having thickness of 1.0 mm.Subsequently, the aluminum alloy plate members were subjected to asolution heat treatment and quenching into water, and then naturallyaged at room temperature for seven days. Thus, there were obtained T4temper members. Two aluminum alloy members with T4 temper formed of eachof the alloys K-T were butt-joined by the friction stir welding (FSW)process under conditions similar to those in the above Example 2 withtheir widthwise end portions butted to each other, thereby producingjoined products. The joined products were subjected to a heat treatmentunder respective conditions indicated in the following TABLE 9 withinone hour after the completion of friction stir welding, therebyproviding test members 81-110. The test members 81-110 were stored at20° C. for seven days, and the Vickers hardness test, the tensile testand the formability test explained above were performed on the testmembers 81-110. The results of the tests are also indicated in TABLE 9.

TABLE 9 Vickers hardness Tensile properties Heat treatment Heat- TensileYield Forming Test Temperature Time Stir affected Base strength strengthElongation limit height member Alloy (° C.) (second) zone zones material(MPa) (MPa) (%) Fractured position (mm) 81 K 120 30 120 111 124 377 29011 Heat-affected zones 12.6 82 K 380 30 99 94 93 305 254 14 Basematerial 14.5 83 K 250 1800 99 98 96 315 246 13 Base material 14.5 84 L120 30 124 109 128 385 280 10 Heat-affected zones 12.7 85 L 380 30 94 9190 296 252 14 Base material 14.1 86 L 250 1800 98 97 95 304 242 14 Basematerial 13.9 87 M 120 30 129 119 135 409 341 14 Heat-affected zones12.5 88 M 380 30 94 90 91 299 258 14 Base material 14.0 89 M 250 1800100 97 98 309 269 13 Base material 14.1 90 N 120 30 56 45 58 158 88 13Heat-affected zones 12.5 91 N 380 30 50 43 43 158 75 13 Base material13.9 92 N 250 1800 57 54 54 181 152 11 Base material 13.4 93 O 120 30 7469 78 229 129 12 Heat-affected zones 13.5 94 O 380 30 64 66 61 209 11213 Base material 14.7 95 O 250 1800 78 76 75 245 211 11 Base material14.3 96 P 120 30 72 66 71 219 128 10 Heat-affected zones 14.1 97 P 38030 60 60 59 198 113 12 Base material 13.7 98 P 250 1800 74 73 73 234 19910 Base material 13.9 99 Q 120 30 75 64 72 218 129 10 Heat-affectedzones 14.2 100 Q 380 30 58 58 53 195 112 11 Base material 13.6 101 Q 2501800 75 76 71 233 198 11 Base material 13.7 102 R 120 30 79 69 80 241144 11 Heat-affected zones 14.2 103 R 380 30 64 63 60 214 117 12 Basematerial 13.9 104 R 250 1800 85 80 80 259 222 11 Base material 13.6 105S 120 30 120 109 125 359 241 10 Heat-affected zones 13.4 106 S 380 30 9692 91 315 196 12 Base material 14.1 107 S 250 1800 73 71 71 224 132 13Base material 13.9 108 T 120 30 126 112 124 389 234 10 Heat-affectedzones 13.8 109 T 380 30 83 84 79 273 158 11 Base material 14.5 110 T 2501800 75 75 75 253 124 12 Base material 14.3

It will be apparent from the results indicated in the above TABLE 9that, in each of the test members 81, 84, 87, 90, 93, 96, 99, 102, 105and 108, the metal structure of the base material portions was notreverted due to the heat treatment temperature lower than the reversiontreatment temperature and the Vickers hardness of the base material washigher than that of the heat-affected zones. Further, fracture occurredat the heat-affected zones having the lowest hardness. Moreover, thesetest members have low values of breaking elongation of less than 16% andlow values of forming limit height of less than 15.0 mm. In each of thetest members 82, 85, 88, 91, 94, 97, 100, 103, 106 and 109, softening byoveraging was caused due to the heat treatment temperature higher thanthe reversion treatment temperature, so that the intended reversionstructure was not obtained. Accordingly, these test members have lowvalues of breaking elongation and forming limit height though fractureoccurred at the base material in the tensile test. In each of the testmembers 83, 86, 89, 92, 95, 98, 101, 104, 107 and 110, softening byoveraging was caused due to the excessively long heat treatment time, sothat the intended reversion structure was not obtained. Accordingly,these test members have low values of breaking elongation and forminglimit height though fracture occurred at the base material in thetensile test.

Comparative Example 6

For comparison, initially, ten aluminum alloys K-T having respectivechemical compositions which are the same as those in the above Example 2were processed in the same manner as in the above Example 2 to providealuminum alloy plate members each having thickness of 1.0 mm.Subsequently, the aluminum alloy plate members were subjected to asolution heat treatment and quenching into water, and then naturallyaged at room temperature for seven days. Thus, there were obtained T4temper members. Two aluminum alloy members with T4 temper formed of eachof the alloys K-T were butt-joined by the friction stir welding (FSW)process under conditions similar to those in the above Example 2 withtheir widthwise end portions butted to each other, thereby producingjoined products. The joined products were stored at room temperature for72 hours after the completion of friction stir welding. Then, the joinedproducts were subjected to a heat treatment under respective conditionsindicated in the following TABLE 10, thereby providing test members111-120. The obtained test members 111-120 were stored at 20° C. forseven days, and the Vickers hardness test, the tensile test and theformability test explained above were performed on the test members111-120. The results of the tests are also indicated in TABLE 10.

TABLE 10 Vickers hardness Tensile properties Heat treatment Heat-Tensile Yield Forming Test Temperature Time Stir affected Base strengthstrength Elongation limit height member Alloy (° C.) (second) zone zonesmaterial (MPa) (MPa) (%) Fractured position (mm) 111 K 250 10 112 108111 379 294 12 Heat-affected zones 12.8 112 L 240 20 113 110 113 385 28313 Heat-affected zones 12.6 113 M 250 20 121 118 122 406 335 12Heat-affected zones 12.2 114 N 220 20 53 50 52 160 85 13 Heat-affectedzones 11.9 115 O 270 10 67 64 66 227 125 13 Heat-affected zones 13.3 116P 250 10 61 58 60 216 123 14 Heat-affected zones 14.1 117 Q 250 5 63 6162 214 122 13 Heat-affected zones 14.2 118 R 270 20 71 70 72 251 134 14Heat-affected zones 14.6 119 S 200 20 113 110 112 359 208 9Heat-affected zones 13.3 120 T 180 30 112 109 110 388 219 10Heat-affected zones 13.7

It is understood from the results shown in the above TABLE 10 that, inany of the test members 111-120, the friction stir welding operation wasperformed on the aluminum alloy members which had suffered fromage-hardening after the friction stir welding, so that the stir zone,the heat-affected zones and the base material have the Vickers hardnessvalues substantially equal to one another and the Vickers hardness ofthe heat-affected zones is slightly smaller than those of the stir zoneand the base material. Accordingly, fracture occurred at theheat-affected zones. Moreover, all of the test members 111-120 have lowvalues of breaking elongation less than 16% and low values of forminglimit height less than 15.0 mm.

1. A method of joining heat-treatable aluminum alloy members by frictionstir welding, comprising the steps of: a T4-treatment-performing step ofperforming a T4 treatment on heat-treatable aluminum alloy members so asto impart T4 temper to the heat-treatable aluminum alloy members; ajoining step of joining the heat-treatable aluminum alloy members withT4 temper by friction stir welding to provide a joined product; and areversion-treatment-performing step of performing a reversion treatment,the reversion-treatment-performing step being carried out prior to orafter the joining step.
 2. The method according to claim 1, wherein thereversion-treatment-performing step is carried out prior to the joiningstep for placing the heat-treatable aluminum alloy members in areversion state and the joining step is carried out such that theheat-treatable aluminum alloy members in the reversion state are joinedtogether by friction stir welding.
 3. The method according to claim 2,wherein the reversion treatment is a heat treatment in which theheat-treatable aluminum alloys with T4 temper are heated at an elevatedtemperature of 150-350° C. for a time period of not longer than 300seconds.
 4. The method according to claim 2, wherein each of theheat-treatable aluminum alloy members is formed of a 6000 seriesaluminum alloy and the reversion treatment is a heat treatment in whichthe heat-treatable aluminum alloy members with T4 temper are heated atan elevated temperature of 200-350° C. for a time period of not longerthan 300 seconds.
 5. The method according to claim 2, wherein each ofthe heat-treatable aluminum alloy members is formed of a 2000 seriesaluminum alloy and the reversion treatment is a heat treatment in whichthe heat-treatable aluminum alloy members with T4 temper are heated atan elevated temperature of 150-300° C. for a time period of not longerthan 300 seconds.
 6. The method according to claim 2, wherein each ofthe heat-treatable aluminum alloy members is formed of a 7000 seriesaluminum alloy and the reversion treatment is a heat treatment in whichthe aluminum alloy members with T4 temper are heated at an elevatedtemperature of 150-250° C. for a time period of not longer than 300seconds.
 7. The method according to claim 2, wherein the joining step iscarried out on the heat-treatable aluminum alloy members in thereversion state before an increase in tensile strength of theheat-treatable aluminum alloy members due to natural aging exceeds 10MPa.
 8. The method according to claim 2, wherein the reversion treatmentis performed using a heating means selected from a salt bath, an oilbath, an air-heating furnace, an iron, infrared heating and inductionheating.
 9. The method according to claim 2, wherein thereversion-treatment-performing step is carried out such that thereversion treatment is sequentially performed using a suitable heatingdevice along portions of the heat-treatable alloy members at which theyare to be joined together by friction stir welding and the joining stepis carried out such that the portions which have been subjected to thereversion treatment are sequentially joined together by friction stirwelding.
 10. The method according to claim 1, wherein thereversion-treatment-performing step is carried out after the joiningstep so as to perform the reversion treatment on the joined productobtained in the joining step, before GP zones are formed at a stir zoneof the joined product.
 11. The method according to claim 10, wherein thereversion treatment is a heat treatment in which the joined product isheated at an elevated temperature of 150-350° C. for a time period ofnot longer than 300 seconds.
 12. The method according to claim 10,wherein each of the heat-treatable aluminum alloy members is formed of a6000 series aluminum alloy and the reversion treatment is a heattreatment in which the heat-treatable aluminum alloy members are heatedat an elevated temperature of 200-350° C. for a time period of notlonger than 300 seconds.
 13. The method according to claim 10, whereineach of the heat-treatable aluminum alloy members is formed of a 2000series aluminum alloy and the reversion treatment is a heat treatment inwhich the heat-treatable aluminum alloy members are heated at anelevated temperature of 150-300° C. for a time period of not longer than300 seconds.
 14. The method according to claim 10, wherein each of theheat-treatable aluminum alloy members is formed of a 7000 seriesaluminum alloy and the reversion treatment is a heat treatment in whichthe heat-treatable aluminum alloy members are heated at an elevatedtemperature of 150-250° C. for a time period of not longer than 300seconds.
 15. The method according to claim 10, wherein thereversion-treatment-performing step is carried out on the joined productobtained in the joining step before a time period during which thejoined product undergoes natural aging that occurs after theheat-treatable aluminum alloy members have been joined by friction stirwelding exceeds 24 hours.
 16. The method according to claim 10, whereinthe reversion treatment is performed using a heating means selected froma salt bath, an oil bath, an air-heating furnace, an iron, infraredheating and induction heating.
 17. The method according to claim 10,wherein the joining step is carried out such that the heat-treatablealloy members are joined together sequentially along portions thereof atwhich they are to be joined by friction stir welding and thereversion-treatment-performing step is carried out such that thereversion treatment is sequentially performed, using a suitable heatingdevice, on the joined product which at least includes a stir zone formedby friction stir welding in the joining step.